THE RELATIONSHIP BETWEEN SOIL INOCULUM DENSITY AND PLANT INFECTION ASA BASIS FOR A QUANTITATIVE BIOASSAY OF VERTICILLIUM-DAHLIAE

Using potato, eggplant and thorn apple as test plants, the relationshi p between soil inoculum density and plant infection was studied as a b asis for the development of a quantitative bioassay of Verticillium da hliae. A linear relationship was demonstrated (P < 0.05) between soil inoculum density and population density on roots for all three test pl ants and for soil inoculum density and population density in sap extra cted from stems for eggplant. Correlation coefficients were higher wit h densities on or in roots (R-2 varying from 0.45 to 0.99) than with d ensities in stems (R-2 varying from 0.04 to 0.26). With eggplant, popu lation densities on/in root and in sap extracted from stems were signi ficantly correlated at 20 and 25 degrees C with Pearson's correlation coefficients of 0.41 and 0.53, respectively. For potato, root coloniza tion was higher at 15 than at 20 degrees C, whereas the reverse applie d to eggplant. Stems of potato were less densely colonized than stems of eggplant. The pathozone sensu Gilligan (1985) was calculated to be <300 mu m, indicating that infection was caused by microsclerotia whic h were located close to the roots. To assess the density of V. dahliae in plant tissue pipetting infested plant sap on solidified ethanol ag ar medium without salts yielded higher densities than using pectate me dium or mixing sap with molten agar. A bioassay for determining effect s of (a)biotic factors on development of V. dahliae in the plant is re commended with eggplants as a test plant, grown in soil infested with 300 single, viable microsclerotia g(-1) soil at a matric potential of -6.2 kPa, and incubated at 20 degrees C for 8 weeks.